Answer
Phenolphthalein is the most suitable, because it changes its color in basic solutions, and the equivalence point of the titration between $KOH$ and $HNO_2$ is in basic solution.
Work Step by Step
$HNO_2$ is a weak acid and $KOH$ is a strong base; therefore, the pH at the equivalence point of their titration will be basic $(pH > 7)$.
Analyzing the 3 given indicators on page 667, the one where the color changes in a basic solution is the most appropriate, so Phenolphthalein is the correct answer.
